Auchincloss dug his heels in to oppose Planter's bid, saying if it were him, "I'd vote for someone else in the Maine Democratic primary."

Graham Platner has been scandal-prone, but is polling ahead of five-term incumbent Sen. Susan Collins (R-ME) in Maine.

Auchincloss's decision to weigh in on the Maine ballot and buck Platner, his party's own nominee, is noteworthy to watch in the swing state.